The Kattegat Island of Anholt: Sea-Level Changes and Groundwater Formation on an Island

Fluctuations in sea level influence the condition of many coastal groundwater aquifers. A rise in sea level can result in seawater intrusion in areas where the groundwater level is near the present sea level, and it may take a long time for the boundary between salt and fresh groundwater to reach equilibrium after a rapid sea level change. In this paper, the present understanding of the palaeo-hydrology of Anholt and its dependence on the past climate and sea level history will be outlined. Anholt has a single unconfined sand aquifer which can easily be modelled. This has proven to be a case in which relatively simple models can describe the processes that take place.
New data are presented which provide a detailed description of the last 16,000 years of climate and sea level change influence on the forces that have formed the island. This geological history can be used to provide information on the history of groundwater recharge and drainage, and the development of the salt-fresh groundwater interface under a sand island. The fact that the center of Anholt was covered by the sea 6,000 years ago, and consequently the freshwater lens, over 100 m below sea level, did not exist means that the present equilibrium between the saltwater and freshwater lens has been established in less than 6,000 years. These results can be used to give guidelines for the future administration of the groundwater resources on Anholt, but hopefully they can also help us understand the dynamics of more complicated coastal groundwater aquifers as on Zealand.